Add tests built-in Instagram provider

...with forged "200 OK" responses in the VCR cassette, so that the test pass despite the legacy Instagram oEmbed endpoint having been deprecated on October 23rd.

lib/oembed/providers.rb

@@ -203,7 +203,7 @@ def add_official_provider(provider_class, sub_type=nil)
 
     # Provider for instagram.com
     # https://instagr.am/developer/embedding/
-    Instagram = OEmbed::Provider.new("https://api.instagram.com/oembed", :json)
+    Instagram = OEmbed::Provider.new("https://api.instagram.com/oembed/", :json)
     Instagram << "http://instagr.am/p/*"
     Instagram << "http://instagram.com/p/*"
     Instagram << "http://www.instagram.com/p/*"

spec/providers/instagram_spec.rb

@@ -0,0 +1,22 @@
+require File.join(File.dirname(__FILE__), '../spec_helper')
+
+describe 'OEmbed::Providers::Instagram' do
+  use_custom_vcr_casette('OEmbed_Providers_Instagram')
+  include OEmbedSpecHelper
+
+  let(:provider) { OEmbed::Providers::Instagram }
+
+  expected_valid_urls = %w(
+    https://www.instagram.com/p/B9bOM-6Ax_d/?igshid=1mn51zsvrhoiq
+    https://www.instagram.com/tv/CCX-gcHArcJ/?igshid=1i0rst4jaz0j
+  )
+  expected_invalid_urls = %w(
+    https://www.instagram.com/u/CCX-gcHArcJ/?igshid=1i0rst4jaz0j
+  )
+
+  it_should_behave_like(
+    "an OEmbed::Providers instance",
+    expected_valid_urls,
+    expected_invalid_urls
+  )
+end